Image forming apparatus and position correcting method

ABSTRACT

An image forming apparatus of an embodiment comprises a printer unit to print a sheet with a plurality of single colors and generate a correction sheet including a plurality of single color regions at different positions between a first reference region and a second reference region. The first and second reference regions are a reference color. An image reading unit is configured to read the correction sheet. A processor is configured to calculate a shift amount in a main scanning direction of the printer unit for each single color region based on RGB luminance values obtained from the correction sheet by the image reading unit and to correct a printer scan position along the main scanning direction for each single color in the plurality of single colors corresponding to the single color regions using calculated shift amounts.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/109,584, filed on Aug. 22, 2018, which is based upon and claims thebenefit of priority from Japanese Patent Application No. 2018-000835,filed Jan. 5, 2018, the entire contents of each of which areincorporated herein by reference.

FIELD

Embodiments described herein relate to an image forming apparatus and aposition correcting method.

BACKGROUND

In an image forming apparatus using a laser scanning unit (LSU) forexposure, color shifting occurs if magnification in a main scanningdirection for each color is different. Therefore, in the related art, astructure is adopted in which alignment sensors are disposed at threeplaces including near the center position along the main scanningdirection. However, such a structure sometimes increases constraints ofcomponent disposition and costs.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an external view of an image forming apparatus according to anembodiment.

FIG. 2 is a view showing an example of a correction sheet.

FIG. 3 is a block view showing a hardware configuration of an imageforming apparatus.

FIG. 4 is a schematic block view of a control unit.

FIG. 5 is a view showing aspects of an example configuration of aprinter unit.

FIG. 6 is a view showing additional aspects of an example configurationof a printer unit.

FIG. 7 is a flowchart of output processing of a correction sheet by animage forming apparatus.

FIG. 8 is a flowchart of image position correction processing by animage forming apparatus.

FIG. 9 shows example of converted RGB values versus position along themain scanning direction.

FIG. 10 shows a table of RGB values according coordinate position.

FIG. 11 shows a difference value table.

FIG. 12 is a graph depicting differences between maximum values and RGBvalues.

FIG. 13 shows a multiplication result table.

FIG. 14 depicts aspects related to calculation processing of atheoretical position.

FIG. 15 shows an image clock modulation setting chart for shift amountfrom theoretical position.

FIG. 16 shows an image clock modulation setting chart for image clockmodulation amount.

DETAILED DESCRIPTION

According to one embodiment, an image forming apparatus includes aprinter unit configured to print a sheet with a plurality of singlecolors and to generate a correction sheet including a plurality ofsingle color regions at different positions along a travel direction ofthe correction sheet in the printer unit. The plurality of single colorregions being between a first reference region and a second referenceregion in the travel direction. The first and second reference regionsbeing a reference color. An image reading unit, such as, for example, adocument scanner, is configured to read the correction sheet. Aprocessor is configured to calculate a shift amount in a main scanningdirection of the printer unit for each single color region based on RGBluminance values of the single color region as obtained from thecorrection sheet using the image reading unit and to correct a printerscan position along the main scanning direction for each single color inthe plurality of single colors corresponding to the single color regionsusing calculated shift amounts.

Hereinafter, an image forming apparatus and a position correcting methodof an example embodiment will be described with reference to drawings.

FIG. 1 is an external view showing an overall configuration example ofan image forming apparatus 100 according to the embodiment.

The image forming apparatus 100 of the embodiment is a multifunctionalperipheral (MFP also referred to as a Multi-Function Peripheral) capableof forming a toner image on a sheet. The sheet is, for example, adocument, or a paper on which characters, images, and the like have beenformed or are to be formed. The sheet may be anything that may be reador scanned by the image forming apparatus 100. The image formingapparatus 100 reads an image on the sheet, then generates digital datacorresponding to the read image and/or an image file storing the digitaldata.

The image forming apparatus 100 includes a display 110, a control panel120, a printer unit 130, a sheet storage unit 140, and an image readingunit 200. The printer unit 130 is a device for forming and fixing atoner image to a sheet.

The display 110 is an image display apparatus such as a liquid crystaldisplay or an organic electroluminescence (EL) display. The display 110displays various information of the image forming apparatus 100. Inaddition, the display 110 outputs a signal corresponding to the inputoperation performed by a user to a processor of the image formingapparatus 100. Further, the display 110 accepts the user's inputoperations.

The control panel 120 has a plurality of buttons. The control panel 120receives the user's input operations. The control panel 120 outputs asignal corresponding to an input operation performed by the user. Thedisplay 110 and the control panel 120 may be configured as an integratedtouch panel.

The printer unit 130 executes image forming processing. In the imageforming processing, the printer unit 130 forms an image on a sheet basedon the image information generated by the image reading unit 200 orreceived via a communication path. The printer unit 130 forms an imageon a sheet using different color toners (e.g., yellow color, magentacolor, cyan color, and black color). The possible image formationinstructions include, for example, an instruction to forma normal image(e.g., print an image according to supplied image information) or aninstruction to forma correction image (e.g., print a test sheet or thelike).

The normal image formation instruction is an instruction for printing animage on a sheet based on the image information generated by the imagereading unit 200 or the like. The instruction to form a correction imageis an instruction for printing an image to be used for correcting theposition or other aspects of the image on the sheet. When receiving aninstruction to form a correction image, the printer unit 130 performsthe following processing, for example: the printer unit 130 forms animage in which a monochrome color patches are interposed betweenreference colors, the monochrome patches being positioned in thevicinity of the center of the sheet along the main scanning direction.The monochrome color patches thus formed can be referred to as acorrection image. The sheet on which an image (i.e., the correctionimage) in which a plurality of monochrome color patches is arranged canbe referred to as a correction sheet.

FIG. 2 is a view showing an example of a correction sheet. As shown inFIG. 2, an image including color patches 51 to 55 is formed as acorrection image on a correction sheet 50. The color patch 51 is a black(K) color patch. The color patch 52 is a magenta (M) color patch. Thecolor patch 53 is a yellow (Y) color patch. The color patch 54 is a cyan(C) color patch. The color patch 55 is a black (K) color patch. Thecorrection image formed on the correction sheet 50 is not limited to theparticular order shown in FIG. 2. On the correction sheet 50, an imagein monochrome color patches are formed between reference color patchesare arranged in order along a sheet carrying direction. In the exampleshown in FIG. 2, the reference color is black (K) color and themonochrome color patches interposed between the reference color patchesare single color M, Y, C patches.

Returning to FIG. 1, the printer unit 130 has a structure in whichalignment sensors are disposed in at least two places offset from thecenter position along the main scanning direction and there is noalignment sensor provided at the center along the main scanningdirection. The two positions of the alignment sensors are, for example,front and rear positions. The sheet storage unit 140 accommodates asheet to be used for image formation in the printer unit 130.

The image reading unit 200 reads images on documents or otherwise asdifferences in light and shade of reflected (or otherwise) light. Forexample, the image reading unit 200 reads an image printed on anoriginal document. The image reading unit 200 reads images with ascanner having a resolution of 300 dots per inch (dpi), for example. Theimage reading unit 200 records the image data as-read by the scanner.The recorded image data may be transmitted to another informationprocessing device via a network. The recorded image data may also beprinted on a sheet by the printer unit 130.

FIG. 3 is a block view showing a hardware configuration of the imageforming apparatus 100. As shown in FIG. 3, the image forming apparatus100 includes a display 110, a control panel 120, a printer unit 130, asheet storage unit 140, an image reading unit 200, a control unit 300, anetwork interface 310, an auxiliary storage device 320, and a memory330. The display 110, the control panel 120, the printer unit 130, thesheet storage unit 140, and the image reading unit 200 have beendescribed above. The control unit 300, the network interface 310, theauxiliary storage device 320, and the memory 330 will be furtherdescribed. Each unit is connected so as to enable data communication viaa system bus 10.

The control unit 300 is, for example, a processor such as a centralprocessing unit (CPU). The control unit 300 controls the operation ofeach functional unit of the image forming apparatus 100. The controlunit 300 executes various kinds of processing by executing softwareprograms and the like. When receiving an instruction to executeprinting, the control unit 300 sends a printing execution instruction tothe printer unit 130. In addition, when receiving an instruction to reada sheet, the control unit 300 causes the image reading unit 200 toexecute a sheet reading.

The network interface 310 exchanges data with other devices across anetwork. Here, the other device is an information processing device,such as a personal computer, for example. The network interface 310operates as an input interface and receives data or instructionstransmitted from other devices. An instruction transmitted from anotherdevice can be an instruction to execute printing, or the like. Inaddition, the network interface 310 operates as an output interface andtransmits data to other devices.

The auxiliary storage device 320 is, for example, a hard disk or asolid-state drive (SSD) and stores various data. In this context,various data includes, for example, an image clock modulation settingtable, digital data, a print job, a job log, and the like. The imageclock modulation setting table is a table used for modulating an imageclock. In this context, the image clock is an image data oscillationclock for causing a laser diode (LD) included in the printer unit 130 toemit light beam at a predetermined timing. The reference image clock isestablished by reference to the two points (positions) of alignmentsensor. In the image clock modulation setting table, the modulationamount (or information corresponding thereto) for the image clock isregistered. The digital data is the image data generated by the imagereading unit 200.

The memory 330 is, for example, a random access memory (RAM). The memory330 temporarily stores data used by a functional unit of the imageforming apparatus 100. The memory 330 may store the digital datagenerated by the image reading unit 200. The memory 330 may temporarilystore any or all of the image clock modulation setting table, thedigital data, the print job, and the job log.

FIG. 4 is a schematic block view showing aspects of the control unit300. The control unit 300 includes an image forming control unit 301, anLD control unit 302, an acquisition unit 303, a conversion unit 304, acenter position calculation unit 305, a theoretical position calculationunit 306, a correction amount determination unit 307, and a correctionunit 308. The acquisition unit 303, the conversion unit 304, the centerposition calculation unit 305, the theoretical position calculation unit306, the correction amount determination unit 307, and the correctionunit 308 can be collectively referred to as an image position correctingunit.

The image forming control unit 301 controls the image forming processingby the printer unit 130. For example, when receiving an instruction toform a normal image, the image forming control unit 301 controls theprinter unit 130 to print an ordinary image. Further, when receiving aninstruction to form a correction image, the image forming control unit301 controls the printer unit 130 to form a correction image.

The LD control unit 302 controls the laser diode in the printer unit130. For example, the LD control unit 302 controls the image clock usedfor generation of the laser light produced by the LD. More specifically,the LD control unit 302 adjusts the positioning of the image in the mainscanning direction by adjusting the image clock by the modulation amountin the image clock modulation setting table. In this manner, the LDcontrol unit 302 may arbitrarily modulate the image clock for each colorto adjust image position in the main scanning direction. In addition,the LD control unit 302 adjusts the image positions so that thepositions of the four YMCK colors match with the positioning of the twoalignment sensors.

The acquisition unit 303 acquires the image data read by the imagereading unit 200 from the auxiliary storage device 320. The acquisitionunit 303 can also acquire the image data of the correction sheet fromthe auxiliary storage device 320.

The conversion unit 304 converts the YMCK monochrome color patches beingformed on the correction sheet into RGB. The conversion unit 304converts the YMCK monochrome color patches into RGB for severalcoordinate positions along the main scanning direction.

The center position calculation unit 305 performs a predeterminedcalculation based on the values of the coordinate position along themain scanning direction and the values of R, G, and B at each coordinateposition. The center position calculation unit 305 first acquires themaximum values of R, G, and B in the values of R, G, and B recorded foreach coordinate position along the main scanning direction. Next, thecenter position calculation unit 305 calculates the difference betweenmaximum values of R, G, and B and the values of R, G, and B for eachcoordinate position. In addition, the center position calculation unit305 calculates the sum of the difference values at each coordinateposition for each of R, G, and B. Further, the center positioncalculation unit 305 calculates the product (multiplication operation)of the values at the coordinate positions of RGB with the differencevalues for R, G, and B at each coordinate position. The center positioncalculation unit 305 then calculates the sum of the product values ofrespective RGB and calculates the center position of a color patch bydividing the sum of the respective products by the sum of the differencevalues for R, G, and B.

The theoretical position calculation unit 306 calculates a theoreticalposition based on a value obtained by dividing the sum of the respectiveproducts calculated by the center position calculation unit 305 by thesum of the respective difference values for R, G, and B. Here, thetheoretical position for the center position of the color patch is theposition as derived by calculation. That is, the theoretical position isnot necessarily the center position of the color patch as actuallyformed on the sheet. Next, the theoretical position calculation unit 306calculates an inclination of the sheet using the center positions forthe two reference color patches to calculate a theoretical position foranother color by taking the inclination of the sheet thus calculatedinto account.

The correction amount determination unit 307 determines a shift amountbetween the theoretical position and the determined center position foreach color patch with the reference color and determines a correctionamount for use in the vicinity of the center along the main scanningdirection with the shift amount. The correction unit 308 selects amodulation setting table according to the determined correction amountand corrects the image position.

Next, aspects of the printer unit 130 will be described with referenceto FIGS. 5 and 6. FIG. 5 is a view showing a specific configuration ofan image forming unit in the printer unit 130. FIG. 6 is a diagramshowing a specific configuration of an exposure device in the printerunit 130.

As shown in FIG. 5, the printer unit 130 includes process units 131(131-Y, 131-M, 131-C, 131-K) for each printer color, a secondarytransfer roller 132, a secondary transfer opposing roller 133, a tensionroller 134, and a transfer belt 135. In FIG. 5, the process units 131corresponding to the respective colors of yellow, magenta, cyan, andblack are distinguished by the designation Y, M, C, and K, respectively.For example, the process unit 131-Y represents the process unit 131 foryellow.

Each process unit 131 forms a toner image corresponding to itsrespective color (YMCK) on the transfer belt 135, which is an endlessbelt. Each process unit 131 respectively includes a photoconductive drum1311 (-Y, -M, -C, -K), a charger 1312 (-Y, -M, -C, -K), an exposuredevice 1313, a developing device 1314(-Y, -M, -C, -K), a photoconductivecleaner 1315(-Y, -M, -C, -K), and a primary transfer roller 1316 (-Y,-M, -C, -K. As noted, in FIG. 5, the units corresponding to therespective colors of yellow, magenta, cyan, and black are designatedwith Y, M, C, and K, respectively. For example, 1311-M represents thephotoconductive drum 1311 in the process unit 131-M (magenta).

In each process unit 131, an electrostatic latent image is generated onthe surface of the photoconductive drum 1311. The photoconductive drum1311 is an image carrier, for example, a cylindrical drum. Thephotoconductive drum 1311 has a photoreceptor substance on the outersurface thereof and has a property of releasing (discharging) a staticelectrical charge at the portions of the surface that are irradiatedwith light. The charger 1312 charges the surface of the photoconductivedrum 1311 with static electricity. The charger 1312 is, for example, aneedle electrode. The exposure device 1313 forms an electrostatic latentimage on the surface of the photoconductive drum 1311 corresponding tothe image to be printed. The exposure device 1313 is, for example, alaser irradiation device. The developing device 1314 supplies toner tothe surface of the photoconductive drum 1311 and develops theelectrostatic latent image with the toner. The photoconductive cleaner1315 removes the residual toner from the photoconductive drum 1311. Theprimary transfer roller 1316 transfers the developed electrostaticlatent image from the surface of the photoconductive drum 1311 to thetransfer belt 135.

The secondary transfer roller 132 transfers the toner image from thetransfer belt 135 to a sheet. The secondary transfer opposing roller 133is disposed at a position facing the secondary transfer roller 132 withthe transfer belt 135 interposed therebetween. The sheet is carriedalong a carrying path 137 and interposed between the secondary transferopposing roller 133 and the secondary transfer roller 132. The tensionroller 134 is a roller for imparting tension to the transfer belt 135.The two alignment sensors 136 are installed inside a pre-secondarytransfer guide below the transfer belt 135. For example, an alignmentsensor 136 is placed at two positions on either side of the carryingpath 137 in the sheet width direction (Z-direction in FIG. 5). That is,in this embodiment, the alignment sensor 136 is not disposed in thevicinity of center along the sheet width direction (Z-direction in FIG.5). At least one of the alignment sensors 136 can have duals functionsas an alignment sensor and a toner adhesion amount sensor, for example.

As shown in FIG. 6, the printer unit 130 includes a polygon mirror 400,a laser diode (LD) 401, an fθ lens 402, a mirror 403, and a beam detect(BD) sensor 404. The polygon mirror 400, the LD 401, the fθ lens 402,the mirror 403, and the BD sensor 404 are configured as an exposuredevice.

The polygon mirror 400 is a mirror having a plurality of reflectingsurfaces (facets). In the present embodiment, a case in which thepolygon mirror 400 has six reflective surfaces will be described as anexample, but, in general, the polygon mirror 400 may have any number ofsurfaces so long as a plurality of reflective surfaces are provided. Thenumber of surfaces on the polygon mirror 400 may be determined byvarious parameters such as intended print speed, resolution, and thelike. For example, the polygon mirror 400 rotates in the direction of anarrow 405 (counterclockwise) by the driving of a motor.

The LD 401 provides laser light at times according to the control of theLD control unit 302. Here, the LD 401 includes a light source for eachcolor of the printer unit 130. Specifically, the LD 401 includes a lightsource for Y color, a light source for M color, a light source for Ccolor, and a light source for K color. The fθ (f-theta) lens 402provides a constant scan rate for the laser lights reflected by thepolygon mirror 400 onto a common imaging plane.

The mirror 403 reflects the laser light reflected by the polygon mirror400 so that the laser light is reflected onto the BD sensor 404. The BDsensor 404 detects the laser light reflected by the mirror 403 as a BDsignal. When detecting the laser light, the BD sensor 404 outputs anotification indicating that the laser light is being detected to the LDcontrol unit 302. The laser light detected by the BD sensor 404 is usedas a scanning start reference signal indicating a scan along the mainscanning direction has started. In addition, the laser light detected bythe BD sensor 404 is synchronized with a writing start position alongthe main scanning direction for each scan line.

FIG. 7 is a flowchart showing a flow of output sheet processing for thecorrection sheet by the image forming apparatus 100. When receiving aninstruction to form a correction image, the control unit 300 controlsthe printer unit 130 to form a correction image (ACT 101). Thereafter,the control unit 300 adjusts the positions so that the positions for thefour YMCK colors match each other at the positions of the two alignmentsensors (ACT 102). Next, the printer unit 130 forms a correction imageon a sheet from the sheet storage unit 140 (ACT 103). For example, theprinter unit 130 forms the correction image shown in FIG. 2. As aresult, the printer unit 130 generates a correction sheet. Thereafter,the control unit 300 controls the internal rollers and outputs thecorrection sheet as generated by the printer unit 130 to the outside ofthe apparatus (ACT 104).

FIG. 8 is a flowchart showing the flow of processing for image positioncorrection by the image forming apparatus 100. When an instruction toread a sheet is received, the control unit 300 causes the image readingunit 200 to execute sheet reading. Under the control of the control unit300, the image reading unit 200 reads the image that has been printed ona sheet (ACT 201). The image reading unit 200 stores the as-read imagedata in the auxiliary storage device 320. The acquisition unit 303acquires the image data of the sheet as-read by the image reading unit200 from the auxiliary storage device 320. The conversion unit 304converts the individual YMCK color patches formed on the correctionsheet into RGB values (ACT 202). FIG. 9 is a view showing an example ofvalues after conversion to RGB. FIG. 9 shows RGB values after theconversion unit 304 has converted the magenta (M) color patch into RGBvalues. That is, an M color patch within the range of the coordinatepositions along the main scanning direction shown in FIG. 9. In FIG. 9,the horizontal axis represents a coordinate position along the mainscanning direction, and the vertical axis represents the RGB luminancevalues. In the example shown in FIG. 9, the RGB values from aboutcoordinate position 1742 to about coordinate position 1752 are lowerthan the RGB values at the other coordinate positions along the mainscanning direction outside this range. The conversion unit 304 generatesa table of values (referred to as “table after RGB conversion”) in whichthe converted value (RGB value) is associated with the coordinateposition.

FIG. 10 is a view showing an example of a table of luminance valuesafter RGB conversion has been performed. In the table shown in FIG. 10,values of R, G, and B are registered in association with severalcoordinate positions along the main scanning direction. FIG. 10 showsthe table after RGB conversion of the M color patch, but the conversionunit 304 also generates additional tables for all the color patches. Theconversion unit 304 outputs the generated table(s) after RGB conversionto the center position calculation unit 305.

The center position calculation unit 305 first acquires the maximumvalues of R, G, and B from the output table after RGB conversion. In theexample shown in FIG. 10, the center position calculation unit 305acquires the luminance value of “230” (see coordinate position=1761) asthe maximum value for R, “237” (see coordinate position=1758) as themaximum value of G, and “238” (see coordinate position=1758) as themaximum value of B. Next, the center position calculation unit 305calculates a difference between each determined maximum value of R, G,and B and the value of R, G, and B at each coordinate position (ACT203).

For example, the center position calculation unit 305 calculates thedifference between the maximum value “230” for R and the value “228” forR at the coordinate position “1732”. In this case, the difference is “2”(in the arbitrary luminance units used in FIG. 9). As described above,the center position calculation unit 305 calculates the differencebetween the maximum value of R and the value of R at each coordinateposition. Similarly, the center position calculation unit 305 calculatesa difference between the maximum value of G and the value of G at eachcoordinate position, and also the difference between the maximum valueof B and the value of B at each coordinate position. The center positioncalculation unit 305 generates a table (hereinafter, referred to as“difference value table”) using the calculated difference values.

FIG. 11 is a view showing an example of a difference value table. In thedifference value table shown in FIG. 11, the difference between themaximum values of R, G, and B and the respective values of R, G, and Bat each coordinated value are registered. In particular, FIG. 11 shows adifference value table for the M color patch, but the center positioncalculation unit 305 also generates difference value tables for everycolor patch on the correction sheet.

Next, the center position calculation unit 305 calculates the sum of thedifference values for each of R, G, and B (ACT 204). That is, the columnof R difference values, G difference values, and B difference values aresummed, respectively. For the example shown in FIG. 11, the centerposition calculation unit 305 calculates the sum “813” for the column ofdifference values for R. Likewise, the center position calculation unit305 calculates the sum “1963” for the column of difference values for G.Additionally, the center position calculation unit 305 calculates thesum “1735” for the column of difference values for B.

FIG. 12 is a graph plotting the differences between maximum values andRGB values according to coordinate position. In FIG. 12, the differencevalues for the M color patch are shown as an example. In FIG. 12, thehorizontal axis represents the coordinate position along the mainscanning direction, and the vertical axis represents the differencebetween the maximum value and the particular coordinate RGB value. Inthe example shown in FIG. 12, the difference values at about coordinateposition “1737” to about coordinate position “1752” are higher than thedifference values at the other coordinate positions outside this rangealong the main scanning direction.

Next, the center position calculation unit 305 multiplies the value ofthe coordinate position along the main scanning direction by thedifference value of R, G, and B at that coordinate position ascalculated in the processing of ACT 203 (ACT 205). An example of thisprocessing will be described with reference to FIG. 11. For example, thecenter position calculation unit 305 multiplies the coordinate position“1732” by the difference value “2” for R at the coordinate position“1732”. In this case, the multiplication result is “3464” (i.e.,1732×2=3464). As described above, the center position calculation unit305 multiples the coordinate position and the difference value of R atthat coordinate position for each coordinate position. Similarly, thecenter position calculation unit 305 calculates the product of thecoordinate position and the difference value of G at that coordinateposition for each coordinate position, and the product of the coordinateposition and the difference value of B at that coordinate position foreach coordinate position. Thereafter, the center position calculationunit 305 generates a table (hereinafter, referred to as “multiplicationresult table”) including these values for the product of the coordinateposition and the each difference value for R, G, and B at eachcoordinate position.

FIG. 13 is a diagram showing an example of a multiplication resulttable. In the multiplication result table shown in FIG. 13, themultiplication results for the value of the coordinate position alongthe main scanning direction and the respective difference values of R,G, and B at that coordinate position are registered. FIG. 13 is amultiplication result table for the M color patch, but the centerposition calculation unit 305 also generates multiplication resulttables for all color patches.

Next, the center position calculation unit 305 calculates the sums thetabulated product values for the column of each of R, G, and B (ACT206). For example, in the example shown in FIG. 13, the center positioncalculation unit 305 calculates the sum “1419353” for the R column ofthe multiplication result table. In addition, the center positioncalculation unit 305 calculates the sum “3427144” for the product valuesin the G column of multiplication result table. In addition, the centerposition calculation unit 305 calculates the sum “3029071” for theproduct values of B column of the multiplication result table.

Then, the center position calculation unit 305 calculates a centerposition of each color patch by using the sum of the difference valuesfor each of R, G, and B and the sum of the products for each of R, G,and B (ACT 207). Specifically, the center position calculation unit 305calculates the center position of a color patch by dividing the sum ofthe products for each of R, G, and B by the sum of the difference valuesfor each of R, G, and B. An example will be described in greater detail.

For example, the center position calculation unit 305 calculates thecenter position=1745.822 for R by dividing the sum “1419353” of theproducts by the sum “813” of the difference values. In addition, thecenter position calculation unit 305 calculates the centerposition=1745.871 for G by dividing the sum “3427144” of the products bythe sum “1963” of the difference values. Further, the center positioncalculation unit 305 calculates the center position=1745.862 for B bydividing the sum “3029071” of the products by the sum “1735” ofdifference values. The center position calculation unit 305 performssimilar center position calculation processing for all the color patcheson the correction sheet.

The theoretical position calculation unit 306 calculates a theoreticalposition based on the center position that was calculated by the centerposition calculation unit 305 (ACT 208). Specifically, first, thetheoretical position calculation unit 306 calculates an inclination ofthe sheet output from the calculated center positions of the tworeference color patches. Then, the theoretical position calculation unit306 calculates a theoretical position of a color patch of another colorby taking this inclination into account. For example, the theoreticalposition calculation unit 306 calculates a theoretical position of theother (non-reference) color patch by a trigonometric function.

FIG. 14 is a view for explaining aspects of the calculation processingfor a theoretical position determination. In FIG. 14, the color patch 51and the color patch 55 are color patches of the reference color (forexample, K). The theoretical position calculation unit 306 calculates aninclination of the sheet based on an assumed straight line 56 connectingthe center positions of the color patch 51 and the color patch 55. Here,if the shift amount of the M color patch 52 relative to reference colorline is ΔM, then the expression ΔM=M(x)−KM(x) sets this value, whereKM(x)=KI(x)−[(KI(y)−M(y))/(KI(y)−Ku(y))x(KI(x)−Ku(x))]. The theoreticalposition calculation unit 306 calculates the shift amount ΔM for thecolor patches of the other colors by a similar calculation. The positionshifted by this calculated shift amount is considered the theoreticalposition.

The correction amount determination unit 307 determines a correctionamount near the center in the main scanning direction based on thecalculated shift amount (ACT 209). For example, the correction amountdetermination unit 307 sets the correction amount so that the shift willbe substantially zero. The correction unit 308 selects the image clockmodulation setting as necessary to correct the image position (ACT 210).

FIGS. 15 and 16 are views showing examples of an image clock modulationsetting table (depicted as a chart in the figures for explanatoryconvenience). In FIG. 15, the horizontal axis represents the number ofsegments along the main scanning direction, and the vertical axisrepresents a shift amount (correction amount) (in millimeters) of themain scanning position. In addition, in FIG. 16, the horizontal axisrepresents the number of segments along the main scanning direction, andthe vertical axis represents a change amount (%) of the image clock. Theimage clock modulation setting table shown in FIG. 15 shows how the mainscanning position fluctuates or varies. The image clock modulationsetting table shown in FIG. 16 shows a modulation amount of the imageclock corresponding to a position change in the image clock modulationsetting table shown in FIG. 15. The image clock modulation setting tablesatisfies the following conditions. For example, the position in thevicinity of the alignment sensor is not changed and, at the other mainscanning positions, the image clock is changed so that the shift amountof the position becomes the maximum in the vicinity of the center. Here,“without changing the position in the vicinity of the alignment sensor”means not to adjust/correct the position of the image in the mainscanning direction.

The correction unit 308 refers to the image clock modulation settingtable shown in FIG. 15 and selects the image clock modulation settingtable corresponding to the determined correction amount. Next, thecorrection unit 308 refers to the image clock modulation setting tableshown in FIG. 16 and acquires the value of the modulation amount of theselected image clock modulation setting table. The correction unit 308corrects the image position for each color by instructing the LD controlunit 302 to modulate the LD 401 with the value of the acquiredmodulation amount.

According to the image forming apparatus 100 configured as describedabove, it is possible to match the position of the image even if thereis no alignment sensor in the vicinity of the center along the mainscanning direction of the image. Specifically, the image formingapparatus 100 first reads the correction image (including a plurality ofcolor patches) formed on the correction sheet. Next, the image formingapparatus 100 calculates a center position of each color patch based onthe RGB values of the correction image as read/scanned. Next, the imageforming apparatus 100 calculates a shift amount based on the centerposition of the color patches of the reference color and the centerposition of another color patch. Then, the image forming apparatus 100determines a correction amount so as to correct the shift in the imageposition. As a result, it is possible to match the position of the imageeven if there is no alignment sensor in the vicinity of the center ofthe image along the main scanning direction. In addition, since thetotal number of alignment sensors may be reduced, it is possible toreduce costs and constraints on component design.

In addition, according to the image forming apparatus 100, even when ascanner with a low resolution is used, it is still possible to correctthe color shift by detecting the shift amount with high accuracy. Inaddition, the image forming apparatus 100 forms a correction image on asheet such that a non-reference color patch is interposed between tworeference color patches. As a result, it is possible to performcorrection in a short time. Therefore, it is possible to performcorrection efficiently.

Hereinafter, a modification example of the image forming apparatus 100will be described. In the modification, the configuration in which thecorrection sheet is read by the image reading unit 200 is shown, but thereading of the correction sheet is not necessarily limited thereto. Forexample, the correction sheet may be read by manually feeding thecorrection sheet.

According to the image forming apparatus 100 of at least one embodimentdescribed above, the apparatus includes a printer unit, an image readingunit, and a processor. The printer unit generates a correction sheet byforming a monochrome correction image to be used for correcting imagepositions of a plurality of colors on a plurality of different kinds ofsheets. The image reading unit reads the correction image formed on thecorrection sheet. The processor calculates a shift amount for thedifferent colors in the main scanning direction based on the RGB valuesof the correction image as read to correct image positions for thevarious colors using a calculated shift amount. As a result, even ifthree alignment sensors are not provided, if at least two alignmentsensors are provided, it is possible to correct the image positions ofthe different colors. Therefore, it is possible to reduce the costswhile suppressing constraints on component design.

Some functions of the image forming apparatus 100 in the above-describedembodiments maybe realized by a computer. In that case, a program forrealizing these functions is recorded on a computer-readable recordingmedium. Then, the functions may be realized by causing a computer systemto read and execute a program recorded on a recording medium in whichthe above-described program is recorded. The “computer system” referredto here includes hardware, and other components such as an operatingsystem and/or peripheral equipment. In addition, “computer-readablerecording medium” refers to a portable medium, a storage device, or thelike. The portable medium is a flexible disk, magneto-optical disk, ROM,CD-ROM or the like. In addition, the storage device is a hard disk builtin the computer system or the like. Furthermore, the “computer-readablerecording medium” may hold a program for a relatively short time suchwhen such a program is transmitted via a network for storage or storedin a volatile memory during execution for a certain period of time. Inaddition, the “computer-readable recording medium” may be a memoryinside a computer system serving as a server or a client. Further, theabove-described program may be realized by combining the above-describedfunctions with a program already recorded in the computer system.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the present disclosure. Indeed, the novel embodiments describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of thepresent disclosure. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the present disclosure.

What is claimed is:
 1. An image forming apparatus, comprising: a printerconfigured to print, on a sheet, a first region of a first color, asecond region of a second color, a third region of a third color, and afourth region of the first color, the second region and the third regionbeing between the first region and the fourth region along a traveldirection of the sheet through the printer, the second color and thethird color being different from each other; an image reader configuredto read the sheet to obtain an image of the sheet; and a processorconfigured to: calculate a first shift amount in a main scanningdirection of the printer for the second region based on the image,calculate a second shift amount in the main scanning direction of theprinter for the third region based on the image, correct a firstprinting position for the second color in the main scanning directionbased on the first shift amount, and correct a second printing positionfor the third color in the main scanning direction based on the secondshift amount.
 2. The image forming apparatus according to claim 1,further comprising: a first alignment sensor at a position away from acenter of the sheet in the main scanning direction, a second alignmentsensor at another position away from the center of the sheet in the mainscanning direction, wherein the processor varies a clock for an exposuresource for the second color to correct the first printing positionbetween the positions of the first and second alignment sensors in themain scanning direction.
 3. The image forming apparatus according toclaim 1, wherein the printer prints the second region and the thirdregion without printing any region with the first color between thesecond region and the third region on the sheet.
 4. The image formingapparatus according to claim 1, wherein the first color is black.
 5. Theimage forming apparatus according to claim 1, wherein the second coloris magenta.
 6. The image forming apparatus according to claim 1, whereinthe second color is yellow.
 7. The image forming apparatus according toclaim 1, wherein the third color is cyan.
 8. The image forming apparatusaccording to claim 1, wherein the first color is black, the second coloris yellow, and the third color is cyan.
 9. The image forming apparatusaccording to claim 1, wherein the image reader is a document scanner.10. The image forming apparatus according to claim 1, wherein theprinter is a laser printer.
 11. A printing apparatus, comprising: aprinter configured to print a test sheet, the test sheet having a firstregion of a first color, a second region of a second color, a thirdregion of a third color, and a fourth region of the first color, thesecond region and the third region being between the first region andthe fourth region along a travel direction of the test sheet through theprinter, the second color and the third color being different from eachother; an image scanner configured to provide an image of the testsheet; and a processor configured to: calculate a first shift amount ina main scanning direction of the printer for the second region based onthe scanned image, calculate a second shift amount in the main scanningdirection of the printer for the third region based on the scannedimage, correct a first printing position for the second color in themain scanning direction based on the first shift amount, and correct asecond printing position for the third color in the main scanningdirection based on the second shift amount.
 12. The printing apparatusaccording to claim 11, wherein the printer comprises a polygonal mirrorconfigured to scan a light source exposure position along the mainscanning direction.
 13. The printing apparatus according to claim 11,further comprising: a first alignment sensor at a position away from acenter of the test sheet in the main scanning direction, a secondalignment sensor at another position away from the center of the testsheet in the main scanning direction, wherein the processor varies aclock for an exposure source for the second color to correct the firstprinting position between the positions of the first and secondalignment sensors in the main scanning direction.
 14. The printingapparatus according to claim 11, wherein the printer prints the secondregion and the third region without printing any region with the firstcolor between the second region and the third region on the sheet. 15.The printing apparatus according to claim 14, wherein the first color isblack, the second color is one of yellow, cyan, and magenta, and thethird color is not black.
 16. A printer apparatus, comprising: a printerconfigured to print an image on a test sheet, the image including afirst region of a first color, a second region of a second color, athird region of a third color, and a fourth region of the first color,the second region and the third region being between the first regionand the fourth region along a travel direction of the test sheet throughthe printer, the second color and the third color being different fromeach other; an image scanner configured scan the image printed on thetest sheet; and a processor configured to: calculate a first shiftamount in a main scanning direction of the printer for the second regionbased on the scanned printed image, calculate a second shift amount inthe main scanning direction of the printer for the third region based onthe scanned printed image, correct a first printing position for thesecond color in the main scanning direction based on the first shiftamount, and correct a second printing position for the third color inthe main scanning direction based on the second shift amount.
 17. Theprinter apparatus according to claim 16, further comprising: a firstalignment sensor at a position away from a center of the test sheet inthe main scanning direction, a second alignment sensor at anotherposition away from the center of the test sheet in the main scanningdirection, wherein the processor varies a clock for an exposure sourcefor the second color to correct the first printing position between thepositions of the first and second alignment sensors in the main scanningdirection.
 18. The printer apparatus according to claim 16, wherein theimage on test sheet includes no region of the first color interposedbetween the second region and the third region.
 19. The printerapparatus according to claim 18, wherein the first color is black, thesecond color is one of yellow, cyan, and magenta, and the third color isnot black.
 20. The printer apparatus according to claim 16, wherein thefirst color is black, the second color is one of yellow, cyan, andmagenta, and the third color is not black.